1. Technical Field
The invention relates to a compact integrated assembly designed for isostatic mounting of a device on a support structure, and allowing corrections to the position of this device along six degrees of freedom.
The term xe2x80x9cisostaticxe2x80x9d means that no stress is generated in the device supported by the mounting assembly.
The mounting assembly according to the invention may be used in many technical fields whenever it is desirable to be able to correct the position and orientation of the supported device with high precision, occupying a restricted volume and without generating any deformation to this device.
One particularly advantageous application relates to optical devices used on spacecraft, and particularly space telescopes.
2. State of the Art
Telescopes include different optical elements such as mirrors and lenses.
The optical performances of telescopes are strongly related to the alignment precisions of the optical elements contained in them. The requested precisions then increase with increasing required performances.
In the case of a space telescope, the optical elements are aligned on the ground.
This alignment is disturbed over time, particularly due to the following phenomena:
residual deformations caused by launching loads (micro-deformations of parts, sliding in links);
ageing of materials;
thermoelastic (cyclic) and hygroelastic effects.
Therefore, during the life of the telescope in orbit, it is desirable to be able to compensate these disturbing phenomena by a precise correction of the position of one or several optical elements of the telescope according to at least five degrees of freedom (in general the rotation of an optical element around its optical axis has no effect on optical performances). This position correction may be controlled by an onboard computer, or by remote control from the ground with a variable frequency and amplitude.
Therefore each optical element concerned is linked to the support structure by a mounting assembly equipped with position correction means.
The mounting assembly must include filter devices if the position correction is to be made without deformation of the optical element.
Furthermore, the mounting assembly must not be on the optical path of the telescope, to avoid blocking the light flux. This would cause a loss of the radiometric performances of the telescope. Therefore, it is important that the mounting assembly occupies the smallest possible volume. In particular, if the optical element is a mirror, this means that the mounting assembly can be placed behind the mirror.
Several different documents have proposed mounting assemblies with position correction means for optical elements in space telescopes.
Thus, document U.S. Pat. No. 4,629,297 proposes a secondary mirror of a telescope supported so that it can be adjusted in five degrees of freedom. The telescope comprises an outside tube, inside which the secondary mirror is installed by radially oriented arms. In the described device, the motors used to make remote controlled corrections to the position and orientation of the secondary mirror are located outside the outside tube. Movements are transmitted between motors and the flexible support of this secondary mirror by rods placed in some of the arms, which are then tubular.
This layout has the disadvantage that it reduces the tube outside diameter for a given overall size. Placing the motors outside the tube means that the tube diameter has to be smaller than it would otherwise be. Instrument performances are thus reduced.
Mounting assemblies using flexible rods placed tangentially between the device to be supported and the support structure are described in a number of documents, particularly document FR-A-2 517 019, document FR-A-2 180 252 and document FR-A-2 724 236. However, none of these assemblies enables position and/or orientation adjustment.
In document U.S. Pat. No. 4,726,671, flexible joints placed at each end of the tangent rods enable relative displacements of a mirror with respect to its support without applying unacceptable stresses on the mirror. However, adjustment possibilities offered. by this type of mechanism are not sufficient to satisfy the needs generated by the disturbances that occur after the adjustment made on the ground.
In document EP-A-0 628 940, a system with multiple jacks is inserted between a floor and platform to simulate flight movements on this platform. The system is connected to the floor at three points and to the platform at three other points. Two jacks are hinged at each of these points. The opposite ends of these jacks are connected to the vertices of a deformable triangle materialized by three other jacks, in order to form a set of adjacent deformable triangles.
Furthermore, document DE-C-41 17 538 proposes installing a plate supporting a telescope and its optical bench on a satellite platform, using three stands, each of which is materialized by two telescopic rods in V-formation, the ends of which are mounted on a ball joint, at three points on the platform and at six points on the plate.
The main purpose of the invention is a compact integrated assembly for mounting a device on a support structure, while allowing automatic position and attitude correction about six degrees of freedom, over a distance of up to a few hundred micrometers in translation and a few hundred microradians in rotation, with a resolution of a few tenths of a micrometer.
According to the invention, this result is achieved using a mounting assembly to mount a device on a support structure, characterized by the fact that it comprises three mounting devices each in the form of a deformable triangle in which a first variable length arm is mounted at its opposite ends on a pair of corresponding anchor parts that can be fixed to the support structure and in which a first vertex opposite the first arm is materialized by a corresponding support part that can be fixed on the device to be mounted.
In a preferred embodiment of the invention, the three mounting devices are identical.
When the device to be supported has an axis of symmetry, the three mounting devices are preferably uniformly distributed about this axis and the planes containing the triangles formed by the mounting devices are approximately tangent to a circle centered on the above-mentioned axis of symmetry.
Furthermore, the other arms of the triangle formed by each mounting device may be materialized either by rigid arms or by variable length arms. For rigid arms, the three arms of the triangle are connected to each other by friction free hinges with zero play.
Advantageously, the friction free hinges with zero play comprise pairs of flexible strips supported at least beyond each end of the other two arms of the triangle formed by each mounting device and aligned with the arms. Each pair of flexible strips then comprises two flexible strips fixed end to end along a longitudinal axis of the arm that they extend, and located in two planes orthogonal to each other and passing through the said longitudinal axis.
The other two arms of the triangle formed by each mounting device are generally connected to the support part through one of the friction free hinges with zero friction.
Preferably, the other two vertices of the triangle formed by each mounting device are materialized by two connecting parts. The other two arms of the triangle are then connected to each connecting part by means of one of the friction free hinges with zero play.
According to a preferred embodiment of the invention, the first arm of the triangle formed by each mounting device is materialized by two variable length arms located side by side approximately parallel to each other. Two first opposite ends of each of these two arms are then connected to two anchor parts that can be mounted on the support structure. The two opposite ends of the two arms cooperate with the two anchor parts, through guide means.
Two first opposite ends of each of these two arms are then connected to two anchor parts that can be mounted on the support structure. The two opposite ends of the two arms cooperate with the two anchor parts, through guide means.
In this case, each connecting part may be fixed to one of the second opposite ends of the arms and guide means are inserted between connecting parts and anchor parts.
Preferably, one of the friction free hinges with zero play is then inserted between the first end of each of the arms and the anchor part to which this first end is connected.
In the preferred embodiment of the invention, the longitudinal axes of the two arms are located in a plane which may be either the plane containing the triangle formed by the mounting device including these arms, or a plane approximately perpendicular to this plane.
According to a first variant embodiment of the invention, the first arm of the triangle formed by each mounting device is materialized by two variable length arms located end to end along a common longitudinal axis. Two first opposite ends of each of these two arms then cooperate with the said pair of anchor parts through guide means and two second adjacent ends of each of two arms are connected to a third anchor part that can be fixed on the support structure.
In this case, each connecting part is preferably connected to one of the first opposite ends of the arms and guide means are inserted between the connecting parts and the pair of anchor parts.
In this first variant embodiment of the invention, one of the friction free hinges with zero play may be inserted either between the third anchor part and the second end of each of the arms, or between the first end of each of the arms and the connecting part to which the arm is connected.
According to a second embodiment of the invention, the first arm of the triangle formed by each mounting device is materialized by a single variable length arm, one first end of which is fixed on a first anchor part of the said pair and on a second end of which cooperates with a second anchor part of the said pair through guide means. The first and second anchor parts are then mounted on the support structure.
In this case, a first connecting part is preferably fixed to the first anchor part. The second connecting part is then fixed to the second end of the single arm, and guide means are inserted between the second connecting part and the second anchor part.
In general, guide means may be either translation friction free guide means, or rotation guide means rotating about an axis perpendicular to a plane containing the triangle formed by each mounting device. In the former case, the guide means advantageously comprise at least one flexible strip perpendicular to the longitudinal axis of the arm with which these guide means cooperate. The flexible strip then connects this arm to the adjacent anchor part.
In a preferred application of the invention, the device to be supported is a space telescope mirror.